This invention relates to a process for preparing open-celled flexible polyurethane foam.
It is well-known to prepare open-celled flexible polyurethane foam by contacting under reaction conditions an organic polyisocyanate with a polyol in the presence of a blowing agent or blowing agent precursor.
Generally, suitable polyols are those having a hydroxyl equivalent weight of from about 800 to about 3000, and suitable polyisocyanates are the aromatic polyisocyanates such as, for example, diphenylmethane diisocyanate, toluene diisocyanate and polymeric polyphenyl polymethane polyisocyanate.
Current commercial trends are to employ a blowing agent precursor, especially water, in preference to a chlorofluorocarbon (CFC) physical blowing agent so providing for the cellular structure of the foam. CFC blowing agents, especially the fully halogenated compounds, are not favored for environmental reasons and are thought to be a contributory factor in the depletion of ozone from the earth's upper atmosphere.
Water functions as a blowing agent precursor by generating in situ the gas, carbon dioxide, through reaction with the isocyanate. The carbon dioxide provides for the cellular structure of the foam by causing the polyurethane polymer to expand.
A frequent observation when using the water/isocyanate reaction as a means of blowing to provide a foam is that the density of the resulting foam is substantially higher than might be anticipated when comparing the gaseous molar volume of the carbon dioxide theoretically produced to an equivalent molar volume of a CFC blowing agent.
A further observation is that polyurethane foams prepared where a significant amount, or all, of the blowing is provided by the water/isocyanate reaction are generally "harder" foams with inferior or undesirable physical properties, including foam discoloration brought about by the increased reaction exotherm of foam preparation in the presence of water.
The observation of increasing foam "hardness" and discoloration is seen in systems where the polyisocyanate is a toluene diisocyanate. Foams prepared with systems where the polyisocyanate consists essentially of 4,4'-diphenylmethane diisocyanate (4,4'-MDI) additionally suffer from the problem of higher densities as already mentioned.
To overcome the problem of higher densities with 4,4'-MDI systems, it is known in the art to prepare foams where the 4,4'-MDI has been blended with quantities, up to 50 weight percent of 2,4'-MDI prior to preparing the foam. Such resulting foams have a lower density for a given quantity of blowing agent/precursor and improved physical properties including "softness". The increase in the "softness" character of the foam is thought to be obtained by reducing the ability of the polymer morphology to develop hard centers of crystallinity. However, the isomer 2,4'-MDI is in relatively short supply and, therefore, is not a commercially attractive route to providing foams.
It is therefore desirable to provide a process for preparing open-celled, flexible, polyurethane foam in the presence of predominantly the blowing agent precursor, water, which overcomes or at least minimizes the problems of foam "hardness", discoloration and higher densities independently of the type of isocyanate employed.
To this purpose, we have investigated the use of phosphorus-containing compounds as catalysts in a process for preparing a flexible polyurethane foam.
The use of phosphorus-containing compounds, particularly those identified as promoting the formation of carbodiimide groups when preparing rigid, hard, polyurethane-modified polyisocyanate foams is known. Publications such as, for example, U.S. Pat. Nos. 3,645,923; 3,717,596; 3,740,709; 3,806,475 and 3,981,829 describe the use of certain phosphorus-containing compounds to enhance properties such as compressive strengths, fire retardancy and reduce friability of rigid foams.
Similarly, the use of certain phosphorus-containing compounds to modify and provide liquid storage-stable polyisocyanates is known and described in, for example, U.S. Pat. Nos. 3,449,256; 3,640,966; 3,670,502; 4,085,146; 4,088,665 and 4,424,288. To modify and provide storage-stable polyisocyanates, the phosphorus-containing compound is typically employed in only from about 1.0 to about 100 ppm by weight of isocyanate. To ensure the storage stability of the resulting polyisocyanate, the phosphorus-containing compound must either be deactivated and/or removed, or at least be inactive at the common storage temperatures of isocyanates.